Until relatively recent times it has been the practice when installing conductors in cross country power lines to hand thread the stringing blocks with a light line. The stringing blocks are suspended from conductor insulators mounted on the supporting towers and it has been necessary for workmen to mount the towers and thread the light line through the throat of each stringing block after which this line is employed to pull in a high strength conductor hauling cable. This initial hand threading operation has been costly, time consuming and hazardous. Later proposals have been made for stringing blocks having slotted frames permitting the hauling line to be advanced along the power line by aerial transport and lowered into the stringing block throat through the frame slot. Patents disclosing such techniques include U.S. Pat. Nos. to Bozeman 4,018,422, and Chadwick 3,905,581. These threading techniques are feasible only if the stringing block is supported outside one side of the tower. However, this limitation has been circumvented by Lederhos et al 4,006,884 wherein there is provided a stringing block threading apparatus utilizing a short length of line known as a threading loop. One of these loops is preassembled through the throat of each stringing block with its ends releasably supported in apparatus located above the tower and operable to uncouple a hauling line connector from an aerially delivered line and substituting the threading loop between the uncoupled ends of the hauling line. The present invention is closely related to the operating principles of the Lederhos et al teachings but incorporates important improvements and certain modified operating principles disclosed hereinbelow.